1. Field of the Invention
The invention relates to the field of fluids containing gas bubbles used in ultrasonic cleaning of substrate surfaces.
2. Description of Related Art
Removal of particulate contaminants from a semiconductor substrate can be accomplished by ultrasonic cleaning. An ultrasonic cleaner, often colloquially referred to as a sonicator, is a cleaning device that uses ultrasound (usually from 150-1,500 kHz) and an appropriate cleaning solution to clean delicate items. When the frequency of ultrasound is close to or above 1,000 kHz (1 MHz) it is often referred to as “megahertz”. Ultrasound is not effective without the cleaning solution, but it enhances the effect of a solution appropriate for the item to be cleaned and the soiling.
In an ultrasonic cleaner, the object to be cleaned is placed in a chamber containing a suitable solution (aqueous or organic compound, depending on the application). In aqueous cleaners, a surfactant may be added to break down the surface tension of the water base. An ultrasound generating transducer built into the chamber, or lowered into the fluid, produces ultrasonic waves in the fluid by changing size in concert with an electrical signal oscillating at ultrasonic frequency. This creates sufficiently energetic compression waves in the liquid of the tank which ‘tear’ the liquid apart, leaving behind many millions of microscopic ‘voids’ or ‘partial vacuum bubbles’ (cavitation). These bubbles collapse with enormous energy: temperatures of 10,000 K and pressures of 350 10^6 Pa have been reported. However, the bubbles are so small that they do no more than clean and remove surface dirt and contaminants. The higher the frequency, the smaller the nodes between the cavitation points, which allows for cleaning of more intricate detail.
In semiconductor wafer cleaning the nucleation of the acoustic cavitation often requires the proper pre-treatment of the liquid and elevated acoustic pressures to achieve the onset of nucleation. Furthermore, this nucleation results in limited nucleation densities from which only a small portion, related to the size distribution, will be active in cleaning. Typically the elevated acoustic pressure will drive bubble oscillations to a more violent regime (transient cavitation), which usually causes damage. The ultrasonic cleaning process is thus often tuned for nucleation and not for its microscopic effects (results of the bubble activity), which is required to avoid any structural damage on, e.g., fragile structures present on the substrate.
There is therefore a need for treatment and cleaning technologies that effectively process or clean with minimal damage to the substrate.